Radiation exceeding government safety limits has seeped into groundwater under a tsunami-crippled Japanese nuclear plant, according to the operator, but experts still claim it is unlikely to contaminate drinking supplies.

The leak is, however, a concern and an indicator of how far the Tokyo Electric Power Co. is from stabilizing dangerously overheating reactors after cooling systems were knocked out in the March 11 tsunami.

TEPCO has increasingly asked for international help in its uphill battle, most recently ordering giant pumps from the U.S. that were to arrive later this month to spray water on the reactors.

The groundwater contamination was found in concentrations 10,000 times higher than the government standard for the plant. The iodine-131, a radioactive substance that decays quickly, was nearly 50 feet (15 meters) below one of the reactors, according to TEPCO spokesman Naoyuki Matsumo.

Seiki Kawagoe, an environmental science professor at Tohoku University, said the radioactive substances were unlikely to affect drinking water, noting that radiation tends to dissipate quickly in the ground, as it does in the ocean.

But there are two ways the iodine could eventually affect drinking water if concentrations were high enough. One is if it were to seep into wells in the area. For now, a 12-mile (20-kilometer) radius around the plant has been cleared, though residents of the area are growing increasingly frustrated with evacuation orders and have been sneaking back to check on their homes.

The other concern is that contaminated water from the plant could seep into underground waterways and eventually into rivers used for drinking water. Tomohiro Mogamiya, an official with the Ministry of Health, Labor and Welfare's water supply division, said that was "extremely unlikely" since groundwater would flow toward the ocean, and the plant is right on the coast.

There are two nearby filtration plants for drinking water, and both have been shut down because they are just inside the exclusion zone. One takes water from the Kido River, to the south, and another takes it from groundwater below Odaka, to the north. Both are several miles (kilometers) from the coast, and therefore on higher ground.

"When people return to the area we will test the water to make sure it is safe," said Masato Ishikawa, an official with the Fukushima prefecture's food and sanitation division.

Radiation concerns have rattled the Japanese public, already struggling to return to normal life after the earthquake-borne tsunami pulverized hundreds of miles (kilometers) of the northeastern coast. Three weeks after the disaster in one of the most connected countries in the world, 260,000 households still do no have running water and 170,000 do not have electricity.

In the latest report of food becoming tainted, the government said Friday that a cow slaughtered for beef had slightly elevated levels of cesium, another radioactive particle. Officials stressed that the meat was never put on the market.

Radioactive cesium can build up in the body and high levels are thought to be a risk for various cancers. It is still found in wild boar in Germany 25 years after the Chernobyl nuclear disaster, making the pigs off-limits for eating in many cases.

Contamination has also affected work at the plant itself, where radioactive water has been pooling, often thwarting the vital work of powering up the complex's cooling systems.

Despite the leaks, TEPCO hasn't had enough dosimeters to provide one for each employee since many were destroyed in the earthquake. Under normal circumstances, the gauges, which measure radiation, would be worn at all times.

TEPCO has repeatedly relaxed safety standards during the crisis in order to prevent frequent violations. That is not uncommon during emergencies.

Though the company has acknowledged that it was initially slow to ask for help in dealing with the nuclear crisis, experts from around the world are now flooding in. French nuclear giant Areva, which supplied fuel to the plant, is helping figure out how to dispose of contaminated water, and American nuclear experts are joining Japanese on a panel to address the disaster.

Hundreds of thousands more people are living in evacuation centers, most because they lost their homes in the tsunami. But others have been forced to leave their houses near the plant because of radiation concerns.

Some residents are growing angry and frustrated with the government and are increasingly violating the bans to return to their homes to gather whatever they can find.

Fukushima officials have put up posters in all evacuation centers urging residents not to violate the cordon, but also are pressing Tokyo to arrange trips in for the residents as soon as possible.

"There is no doubt in my mind that it is dangerous in there," said Kazuko Hirohara, a 52-year-old nurse from Minami Soma. "I just wish they would have thought about safety before they ruined our lives."